The disclosure relates to the field of the extrusion of plastics materials and, more particularly, rubber-based materials.
Traditionally, these materials are shaped using an extrusion means comprising a threaded endless screw, rotated inside a cylindrical barrel and opening onto profiling means.
In order to improve the characteristics of the products obtained, numerous adaptations have been made to extruder design, more particularly extruder screw designs. Thus, as is known, there is a feed zone, intended to receive the materials in a solid or not very viscous state, followed by a working or plasticizing zone in which the pressure and temperature of the material are raised so that it can be transferred downstream of the device, a homogenization zone in which the material is kneaded in order to ensure that its properties are suitably uniform, and a final part opening into an extrusion die or into a shaping device such as a mold. Most of the energy supplied to the material comes from the mechanical energy transmitted by the extrusion screw which is converted into heat energy under the effect of the shearing that the material experiences as it passes through the various working zones listed hereinabove.
The disclosure is more particularly concerned with the homogenization zone which aims to render the rheological characteristics such as the temperature and fluidity throughout the outgoing material isotropic.
Solutions, notably those described in documents U.S. Pat. No. 3,164,375. GB 842 692, U.S. Pat. No. 3,102,694 or JP S55 32630, in which the barrel of the extruder comprises flights which are set in rotation with respect to the flights of the rotary screw, are known. The flights of the screw and of the barrel are continuous and are the vertices thereof are spaced radially along the entire length of the extruder. Because of this arrangement of the flights, as the screw and the barrel rotate, the stream of material flowing between the flights of the screw is mixed with that of the flights of the barrel, without, however, it being possible for high shearing forces to be applied to the blend.
In order to remedy this problem, other documents describe this part of the extrusion tool as comprising pins, of the type described in publication U.S. Pat. No. 6,709,147, or fixed knives, proposed by way of example in publication FR 2 580 984, opening radially into the internal space of the barrel and collaborating with the interrupted screw flights.
The shape and orientation of the knives or of the flight elements have the essential aim of subdividing and recombining the streams of material a great many times as explained in detail in publication DE 31 50 757 or publication DE 38 05 849. The homogenization zone therefore comprises a succession of flight element stages, advantageously in the form of blades and which are borne respectively by the screw and by the barrel. The flight elements borne by the screw are configured in such a way as to leave empty annular spaces in which the stationary flight elements arranged on the internal wall of the barrel run.
However, it is found that the amount of energy conferred upon the rubber blend at the homogenization zone remains high, and is not always suited to the type of material that it is designed to use.
One solution then is to make the pins radially mobile, as described in publication U.S. Pat. No. 4,199,263, so that the number of pins, and therefore the level of work obtained, can be adapted to suit the nature of the blend. One and the same extruder can then be configured simply on each change of production sequence or during one same production sequence, to allow it to work very different blends such as blends based on synthetic rubbers or blends based on natural rubbers.
Nevertheless, the extrusion screw, the shape of which is designed to accept a great many pins, remains in place inside the barrel between two production sequences. Further, the amount of energy conferred upon the blend may still prove to be too high when the composition of the material is sensitive to a rise in temperature or when the material does not require a very high degree of homogenization.